Cathode structure for electron-discharge devices



ROBERT ADLER IN V EN TOR.

HIS ATTORNEY Sept. 1, 1953 R. ADLER CATHODEI STRUCTURE FOR ELECTRON-DISCHARGE DEVICES Filed April 11, 1950 .BEE.

ilillilfllllll ,II.

Patented Sept. 1 1953 UNITED STATES PATENT OFFICE Robert Adler, Northfield, 111., assignor to Zenith Radio Corporation, a corporation of Illinois Application April 11, 1950, Serial No. 155,208

10 Claims. (Cl. 313-302) This invention relates to electron-discharge devices and more particularly to improved electrodes or other elements for incorporation in such devices.

In the processing of a thermionic oxide-coated cathode, it is conventional practice to coat a conductive sleeve with carbonates of barium, strontium, calcium, and/or similar metals and subsequently to subject the coated cathode sleeve to intense heat (usually 900 C. or higher) in order to decompose the carbonates into the corresponding oxides and to drive off carbon dioxide gas. This operation is generally performed while the device is connected to a vacuum pump so that the carbon dioxide gas which is driven off as a result of decomposition of the carbonates is evacuated from the tube; consequently, the operation is conventionally referred to as the outgassing operation. In subsequent processing, the tube may be completely evacuated and gettered in a conventional fashion, and the oxide-coated cathode so formed may be seasoned in a well-known manner until such time as full electron-emissivity is obtained.

By far the greatest number of high-vacuum oxide-cathode electron tubes produced at the present time comprise internal resistance elements associated with the cathode sleeve for energizing the cathode during operation of the tube. Internal heater elements of this type furnish a very convenient means for elevating the temperature of the carbonate-coated cathode sleeve to a sufficient extent to provide complete outgassing of the carbonates. Thus, in outgassing tubes of this type, the internal indirectheater element is provided with sufiicient current to raise the temperature of the cathode sleeve high enough to satisfy the outgassing requirements.

There are other high-vacuum electron-discharge tubes known to the art, however, which comprise indirectly-heated thermionic cathodes not provided with internal heater resistance elements. For example, there is disclosed and claimed in the copending application of Robert Adler, Serial No. 129,554, filed November 26, 1949, for Electron-Discharge Devices and Circuits, and assigned to the present assignee, a novel electron-discharge device having a bi-directional electron-discharge path. Briefly, such a bilaterally conductive device comprises a pair of thermionic cathodes disposed at opposite ends of a common electron-discharge path; a control grid is provided between the cathodes for controlling the conductivity of the device. In its preferred application as a switch tube in a sawtooth current generator, voltage pulses of large amplitude are produced between the cathodes. Consequently, in order to avoid the necessity for providing special high-voltage insulation, it is preferred that one of the cathodes be energized in operation by means other than a conventional internal resistance heater element. For example, a preferred construction of such a tube may comprise a bipartite outer cathode and a second or inner cathode disposed between the component parts of the outer cathode and substantially surrounded by a control grid. Each part of the outer cathode may be provided with an associated resistance element for heating, while the inner cathode is not provided with such an energizing element but instead is adapted to be energized in operation partly by radiant heat from the outer cathode and partly by heat developed by plate dissipation at the inner cathode in its role as anode during the portion of each cycle in which it operates at a higher potential than the outer cathode.

Since the inner cathode of a switch tube of the type discussed is not provided with an associated internal heater resistance element, some other means must be employed to elevate the temperature of the inner cathode during the processing of the tube in order to provide complete outgassing of the carbonates. It is not feasible to heat the inner cathode during the outgassing operation by electron bombardment from the outer cathode because this has been found to be too destructive of the thermionic coating. Moreover, raising the temperature of the inner cathode to an extent sufficient to provide complete outgassing of the carbonates by induction heating by eddy current loss has been found to have detrimental efiects, such as warpage and the like, upon the control grid surrounding the inner cathode. It is therefore apparent that conventional outgassing methods are not practical for use in the processing of a switch tube of the type under consideration.

It is an important object of the present invention to provide a new and improved element for an electron-discharge device.

It is a further object of the invention to provide an improved electrode for an electron tube.

Yet another object of the invention is to provide a new and improved thermionic cathode.

Still another object of the invention is to provide a bilaterally conductive switch tube of the type disclosed in the above-identified Adler application comprising a new and improved inner 3 cathode which may be readily processed without undesirably affecting the other tube elements.

In accordance with the present invention, as specifically applied to a switch tube of the type discussed above, the inner cathode is constructed at least in part of a ferromagnetic material having a Curie point higher than thatof iron. Preferably, the ferromagnetic materialv used is a ferro-cobalt or a nickel-cobalt alloy, and the Curie point of the alloy selected is preferably above 900 C. However, in its broader aspects, the invention also contemplat'es'the use -of such a ferromagnetic material in the construction of any electrode or other element. having, a solid cross-section in a high-vacuum electron-dis charge device. 1

The features of the presentinvention which are believed to be novel are set forthwith par-.

tases thereo may besthe un er oqd owe e by reference to the following description consider d. in co iun o wi he, c m ny n drawing, in which the single f gure is acrossse t qn i wofi an ele r n-d cha ge device comprising a thermionic cathode constructed in a d nc i h he s ntin n a *Inthe drawing abilaterally conductive electron discharge device comprises a plurality of elements supported within an evacuated envelepe ID in ,convenient'manner well-known to the art. Thedevice comprises a bipartite outer cathode, consisting of two component parts i! and I2respectivelyprovidedwith oppositely disposed electron-emissive coatings l3 and I i. A second or inner cathode i5 is disposed between parts-l l and I2 of the bipartiteouter cathode and is; substantially surrounded. by a control grid it which may conveniently be constructed as a helical winding about a pair of side-rods or supporting. posts I1 and. It," although the specific construction of the control grid is not material to the present-invention. Component parts ll and I 2, of the. outer-pathetic are, individually provided with heater resistance elements i9 and insulatedly supported internally of the respective composite parts ll and 12, and a pair of heat shield-members 2i; and 2 2 are arranged to reflectradiantheat from the outer cathode inwardly in order toenergize the inner cathode l5 c l ur,ir 1g operation of the tube. Heat-radiating finsv 2 3. and 24 are affixed respectively to grid side-rods l1 and [8 to prevent over-heating of the. gridyvires. {Io this. end, the outer surfaces of radiating fins Band 24 are preferably blacken ed to provide effi cient heat radiation, while the ,inner surfaces are highly polished to prevent substantial. radiation inwardly toward the central structure.

he 'nnercatho'de [5 "may conveniently be constructed by forming-a pair of thermionically emissivecoatings 25 and, 26 oniopposite surfaces of a suitable cath de sleeve member 27. No internal, heaterresi's'tance element is provided for inner cathodefsleevemember 21; rather, in accordance with the.v invention, a ferromagnetic slug or insert, 2 B isQ'dispQsed Within cathode sleeve 21. Insert. 28 is constructed of a ferromagnetic material havinga CurieIpoint higher than that of iron (about 760 C.) and preferably above 900 C. Suitable materials for this purpose include certain I ferro cobalt alloys, and a ferrocobalt-alloyhaving a cobalt content of from 30% to by weight is preferred. Small amounts of vanadium, chromium, or other metals may be incorporated in the alloys to provide ductility and other desirable metallurgical properties. One material which has been found particularly suitable is a ferro-cobalt alloy having a cobalt content of substantially 35% and including about 1 or 2% vanadium; this material is commercially available under the trade names Permendur and Hiperco and has a Curie point of about 950 C. Nickel-cobalt alloys having a cobalt content greater than 40% by weight also have Curie pointshigher than that of iron and henceare suitable for the purposes of the invention. Pure cobalt is ferromagnetic and has a Curie point of about 1130 C. and is therefore also suitable for use in accordance with the invention; however, pure cobalt is very diificult to work, in a metallurgical sense, so that ferrocobalt lor'nickel-cobalt alloys having somewhat lower Curie points but having more desirable metallurgical properties are preferred. As an alternative, it is apparent that theinner cathode 55 may be formed with the ferromagnetic insert 28 as a base member. Thus, member 28 may be provided with a nickel plating or the like to which thermionically emissive coatings 25 and may readilyiadhere. In any event, ferromagnetic member 28 may be viewed as a" structural part of the inner cathode E5.

in the processing of the illustrated device, the inner cathode sleeve member 2'! is provided with suitable carbonate coatings in a manner well known in the art, and the tube elementsaire placed within envelope E0. The tube is then con nected to a vacuum pump or evacuating device (not shown). 'Outgassing' of the carbonate coatings on the inner cathode sleeve member fl'l'is readily accomplished by providing an 'felectromagnetic induction heating'field in a direction perpendicular to thejplane of the drawing; any conventional induction heating apparatus (not shown) may be used for this purpose'one particularly suitable device comprising a helical coil which may be disposed coaxially about tube envelope i0 and which is driven from a suitable high-powered source to provide the desired field intensity. I v V The provision of an 'aXia1 heating field results in a rapid increase in the temperature of ferromagnetic insert; 28, due to the hysteresis loss of the material. 'As'thefternperaturebf the insert 28 approaches the Curie point, the hysteresis loop shrinks, so that the energy dissipation per cycle due toh'ysteresis loss decreases. Attemperatures above the Curie point, the material behaves substantially like "any'p'aramagnetic material; in other words. at'such temperatures there is no energy dissipation due to hysteresis loss. Because the direction 'of the induction heating field is substantially parallel to the flat surfaces of the inner cathode, the'amou'nt of eddy current heating in the ferromagnetic insert 28 is so small as to be.practically]negligible. ConsequentlV, the

ferromagnetic insert behaves substantially as its own thermostat, rising to a temperature near its Gurie point in response to an applied axial induction heating, field and maintaining its ternperature near the Curie point so long as the field is of sumcient intensity. I

l'eating of the innercathode during the outgassing operation by hyteresis loss is preferred over eddy current heating for "the reason that control grid i6 is alsoelevated in temperature by eddy current loss in "the'gri d structure. For a given flux density, hysteresis loss is proportional to the frequency of the induction heating field. while eddy current loss is proportional to the square of the frequency. Thus, by maintaining the frequency of the heating field at a relatively low value (but high enough that heat is roduced at the rate required), and by constructing the control grid of non-magnetic or paramagnetic material, a high flux density may be used without over-heating control grid 16. A frequency of the order of l megacycle has been found quite acceptable in practice.

Thus, the present invention provides an improved thermionic cathode of the heaterless variety which may, for example, be advantageously incorporated in a bi-directional electrondischarge device of the type disclosed and claimed in the above-identified Adler application. Although the invention affords exceptional advantage in this environment, it is also contemplated that other solid electrodes or elements of the internal structure of this or of other types of electron-discharge device may be constructed of a ferromagnetic material having a Curie point higher than that of iron, in order to facilitate heating such electrode or element by an induction heating field, in accordance with the invention. For example, it may be desirable in some instances to construct grids, anodes, or other cold electrodes of such material, in order that such electrodes may readily be freed of occluded gases during the processing of the tube.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. An electron-discharge device element having a solid cross-section and constructed at least in part of a ferromagnetic material having a Curie point higher than that of iron.

2. An electron-discharge device electrode having a solid cross-section and constructed at least in part of a ferromagnetic material having a Curie point higher than that of iron.

3. An electron-discharge device element having a solid cross-section and constructed at least in part of a cobalt alloy having a Curie point higher than that of iron.

4. An electron-discharge device element hav ing a solid cross-section and constructed at least in part of a ferro-cobalt alloy having a Curie point higher than that of iron.

5. An electron-discharge device element hav ing a solid cross-section and constructed at least in part of a ferro-cobalt alloy having a cobalt content of from 30% to by weight and having a Curie point above 900 C.

6. An electron-discharge device element constructed at least in part of a nickel-cobalt alloy, the cobalt comprising more than 40% by Weight of the alloy, said alloy having a Curie point higher than that of iron.

7. An electron-discharge device element constructed of substantially pure cobalt.

8. A thermionic cathode comprising: a hollow sleeve; a thermionically emissive coating on at least one surface of said sleeve; and a ferromagnetic insert for said sleeve having a solid crosssection and constructed of a material having a Curie point higher than that of iron.

9. An electron-discharge device comprising: a bipartite thermionic cathode; indirect-heater means for said bipartite cathode; and a heaterless thermionic cathode, disposed between the composite parts of said first-mentioned cathode and substantially surrounded by a control grid, constructed at least in part of a ferromagnetic material having a Curie point higher than that of iron.

10. An electron-discharge device comprising: a bipartite thermionic cathode; indirect-heater means for said bipartite cathode; and a heaterless thermionic cathode, disposed between the composite parts of said first-mentioned cathode and substantially surrounded by a control grid, constructed at least in part of a ferro-cobalt alloy having a cobalt content of from 30% to 50% by weight and having a Curie point above 900 C.

ROBERT ADLER.

References Cited in the file of this patent UNITED STATES PATENTS Number 

10. AN ELECTRON-DISCHARGE DEVICE COMPRISING: A BIPARTITE THERMIONIC CATHODE; INDIRECT-HEATER MEANS FOR SAID BIPARTITE CATHODE: AND A HEATER LESS THERMIONIC CATHODE, DISPOSED BETWEEN THE COMPOSITE PARTS OF SAID FIRST-MENTIONED CATHODE AND SUBSTANTIALLY SURROUNDED BY A CONTROL GRID CONSTRUCTED AT LEAST IN PART OF A FERRO-COBALT ALLOY HAVING A COBALT CONTENT OF FROM 30% TO 50% BY WEIGHT AND HAVING A CURIE POINT ABOVE 900* C. 